polymers

Review Application of Zwitterions in Forward Osmosis: A Short Review

Yu-Hsuan Chiao 1,2,3,† , Arijit Sengupta 4,5,†, Micah Belle Marie Yap Ang 3 , Shu-Ting Chen 2, Teow Yeit Haan 6,7, Jorge Almodovar 2 , Wei-Song Hung 1,3,* and S. Ranil Wickramasinghe 2,6,7,*

1 Advanced Membrane Materials Research Center, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; [email protected] 2 Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701, USA; [email protected] (S.-T.C.); [email protected] (J.A.) 3 R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Chung Li 32023, Taiwan; [email protected] 4 Bhabha Atomic Research Centre, Radiochemistry Division, Mumbai 400085, India; [email protected] 5 Homi Bhabha National Institute, Mumbai 400012, India 6 Research Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor Darul Ehsan, Malaysia; [email protected] 7 Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor Darul Ehsan, Malaysia * Correspondence: [email protected] (W.-S.H.); [email protected] (S.R.W.) † The authorship is equally contributed.

Abstract: Forward osmosis (FO) is an important desalination method to produce potable water. It was also used to treat different wastewater streams, including industrial as well as municipal wastewater. Though FO is environmentally benign, energy intensive, and highly efficient; it still suffers from four types of fouling namely: organic fouling, inorganic scaling, biofouling and colloidal



 fouling or a combination of these types of fouling. Membrane fouling may require simple shear force and physical cleaning for sufficient recovery of membrane performance. Severe fouling may Citation: Chiao, Y.-H.; Sengupta, A.; need chemical cleaning, especially when a slimy biofilm or severe microbial colony is formed. Ang, M.B.M.Y.; Chen, S.-T.; Haan, T.Y.; Almodovar, J.; Hung, W.-S.; Modification of FO membrane through introducing zwitterionic moieties on the membrane surface Wickramasinghe, S.R. Application of has been proven to enhance antifouling property. In addition, it could also significantly improve Zwitterions in Forward Osmosis: A the separation efficiency and longevity of the membrane. Zwitterion moieties can also incorporate Short Review. Polymers 2021, 13, 583. in draw solution as electrolytes in FO process. It could be in a form of a monomer or a polymer. https://doi.org/10.3390/polym13040583 Hence, this review comprehensively discussed several methods of inclusion of zwitterionic moieties in FO membrane. These methods include atom transfer radical polymerization (ATRP); second Academic Editor: Antonio Zuorro interfacial polymerization (SIP); coating and in situ formation. Furthermore, an attempt was made to understand the mechanism of improvement in FO performance by zwitterionic moieties. Finally, the Received: 5 February 2021 future prospective of the application of zwitterions in FO has been discussed. Accepted: 12 February 2021 Published: 15 February 2021 Keywords: zwitterion; forward osmosis (FO); draw solution; antifouling; antibacterial

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Water scarcity has become one of the major global concerns of our time. With the exponential economic development and population growth, water resource management is important [1,2]. Membrane technology is an essential technique for industrial application in water treatment since the appearance of Loeb-Sourirajan membranes on the market in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 1961 [3,4]. This is due to the great advantages of membrane technology such as continuous This article is an open access article separation processes, low energy consumption, small footprint, ease of scale-up, and so distributed under the terms and on [5]. Various driving forces such as pressure, temperature, electrical potential, and conditions of the Creative Commons concentration have been investigated in the past half-century. In the recent decade, forward Attribution (CC BY) license (https:// osmosis (FO) has attracted the attention of various applications like desalination [6–11], creativecommons.org/licenses/by/ wastewater treatment [12–14], food processing [7,15,16], organic solvent separation [17], 4.0/). power generation [18–20], and pharmaceutical industry [21,22].

Polymers 2021, 13, 583. https://doi.org/10.3390/polym13040583 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 583 2 of 18

FO is a sustainable membrane process which adopts osmotic pressure difference to be the driving force across the membrane barrier [23,24]. The permeate is driven from the low osmotic pressure feed solution (FS) side to the high osmotic pressure draw solution (DS) side. It is interesting to note that unlike the relatively high energy cost needed for a pressure- driven reverse osmosis (RO) membrane, the osmotic-driven process of FO only requires minimal electric energy usage for pumping the flow operation [25,26].Because of low hydraulic pressure in FO [27], FO has been reported to have lower fouling tendency, easier foulant removal [28,29], and higher water recovery [2,13]. However, some investigators have indicated that this may not always be the case [25]. These observations suggest that the degree of fouling is dependent on the feed properties. Nevertheless, the high FO flux reversibility advantage was also evaluated in their study where it is stated the FO is a resilient membrane [25]. FO still has a great promise for application in water treatment in the future. Despite the minimal energy demand for FO, there are several challenges to consider in the development of FO: concentration polarization (CP), DS recovery, and fouling deposition in case of a high fouling feed stream [30]. CP can be divided into two types: internal concentration polarization (ICP) and external concentration polarization (ECP) [31]. The ECP could be mitigated by changing the operation condition such as the crossflow velocity and the spacer and module channel design, because it occurs on both side of the membrane surface [32]; by contrast, membrane support layer influences the ICP, so the solution is to modify the membrane structure such as porosity, tortuosity, and thickness to minimum the effect of ICP [30]. Electrospinning technology has been employed to be a substrate to decrease the ICP effect because of high porosity and low tortuosity properties of its unique hierarchical re-entrant textile structure [33,34]. Membranologists have been investigating the membrane formation to create vertically oriented porous substrates (VOPSs) for minimizing or eliminating the ICP effect [35]. To make practical operation feasible, DS recovery is the other challenge to FO. Elimelech and coworkers used a water-soluble mixture of NH3 and CO2 containing ammonium bicarbonate (NH4HCO3) as a thermoresponsive draw solution, which resulted in the recovery at 60 ◦C thereby showing its potential for development in a large-scale facility [36]. Later, different novel draw solutes have been synthesized. For instance, magnetic-responsive nanoparticles, thermo-responsive materials and stimuli-responsive polymer hydrogels [2]. Apart from that, other membrane processes like membrane distillation (MD) and nanofiltration (NF) have been investigated to regenerate the draw solution continuously [37]. Fouling mitigation is a major issue for most membrane separation applications [38–42]. In FO, the degree of fouling is not only related to the feed composition and characteristic of the foulant, but also the hydrodynamic conditions, DS composition, and membrane properties and orientation [43]. Typically, organic fouling, inorganic scaling and biofoul- ing have mainly attracted extensive attention and investigation. For membranologists, the main solution to mitigate the fouling deposited on the membrane is to modify the membrane properties or structure. A wide range of antifouling materials could be used to enhance the antifouling property. Graphene oxide (GO), carbon quantum dots (CQDs), halloysite, zeolite, nanoparticles, hydrophilic monomer, or polymer, and so on were used to modify the FO membrane. Modification using these materials revealed excellent results in mass transport performance and antifouling test. Recently, the anti-biofouling of the FO membrane has been investigated and reported. Hu and coworkers mentioned that dopamine-assisted silver nanoparticles(Ag NPs) coating on the membrane surface reveals that biofilm is significantly thinner than the pristine membrane and 97% lower number of Pseudomonas aeruginosa attached in the Ag NPs coated FO membrane [44]. Throughout the history of developing antifouling materials, they could be divided into three generations, 2-hydroxyethyl methacrylate (HEMA)-based, PEGylated-based, and zwitterionic-based materials. HEMA-based material has rich -OH moieties to generate a tight hydration layer reducing the hydrophobic foulant approach to the layer, but their antifouling property may be lost once it contacts with complex media like human blood Polymers 2021, 13, 583 3 of 19

Throughout the history of developing antifouling materials, they could be divided into three generations, 2‐hydroxyethyl methacrylate (HEMA)‐based, PEGylated‐based, Polymers 2021, 13, 583and zwitterionic‐based materials. HEMA‐based material has rich ‐OH moieties to gener‐ 3 of 18 ate a tight hydration layer reducing the hydrophobic foulant approach to the layer, but their antifouling property may be lost once it contacts with complex media like human blood serum and plasma [45]. The complex physicochemical interactions between the pro‐ tein and the surfaceserum can and affect plasma their [45 fouling]. The complex behavior physicochemical as well. The second interactions generation between is a the protein PEG‐based or andoligo the (ethylene surface glycol) can affect (OEG) their material fouling and behavior it demonstrated as well. The an second excellent generation be‐ is a PEG- havior to avoidbased protein or oligo and cell (ethylene adhesion glycol) onto (OEG) the interface material owing and it to demonstrated the close hydration an excellent behavior layer around OEGto avoid chain. protein The andmost cell significant adhesion disadvantage onto the interface is its owingchemical to thescalability, close hydration layer when oxygen andaround transition OEG chain.metal ions The were most found significant in biochemical disadvantage environment is its chemical [46]. Both scalability, when HEMA and PEGoxygen materials and transition have shown metal a mediocre ions were antifouling found in biochemical performance environment once the pos [46‐]. Both HEMA itively chargedand foulants PEG materials are in contact have though shown a a good mediocre fouling antifouling resistance performance to the negatively once the positively charged pollutecharged because foulants of an electrostatic are in contact repulsion though a[47]. good Thus, fouling zwitterionic resistance‐based to the mate negatively‐ charged rial has graduallypollute attracted because researcher’s of an electrostatic attention, repulsion and therefore [47]. regraded Thus, zwitterionic-based as third‐gener‐ material has gradually attracted researcher’s attention, and therefore regraded as third-generation ation antifouling material. antifouling material. Zwitterionic materials are promising candidates to enhance surface hydrophilicity Zwitterionic materials are promising candidates to enhance surface hydrophilicity and and augment both antifouling and antibacterial properties of the membrane because it augment both antifouling and antibacterial properties of the membrane because it could could create a strong hydration layer through electrostatic and hydrogen bond interaction create a strong hydration layer through electrostatic and hydrogen bond interaction [48–52]. [48–52]. The typical zwitterionic material systems can be separated into phosphobetaine The typical zwitterionic material systems can be separated into phosphobetaine methacrylate methacrylate (PBMA), sulfobetaine methacrylate (SBMA), and carboxybetaine methacry‐ (PBMA), sulfobetaine methacrylate (SBMA), and carboxybetaine methacrylate (CBMA) [53,54], late (CBMA) [53,54], shown in Figure 1. Similarly, they are comprised of cations and ani‐ shown in Figure1. Similarly, they are comprised of cations and anions on the same side chain, ons on the same side chain, which could prevent surface adhesion by either positive or which could prevent surface adhesion by either positive or negatively charged foulants. The negatively charged foulants. The zwitterionic phoshatidycholine displays anti‐biofouling zwitterionic phoshatidycholine displays anti-biofouling ability and biocompatibility to protect ability and biocompatibility to protect cell membranes, consequently, it could theoreti‐ cell membranes, consequently, it could theoretically be used its concept to fabricate or design cally be used itsa related concept antifouling to fabricate membrane or design or a interfaces. related antifouling membrane or inter‐ faces.

FigureFigure 1. The 1. The chemical chemical structures structures of antifouling of antifouling zwitterionic zwitterionic material, material, PBMA, PBMA, SBMA, SBMA, and CBMAand CBMA systems. systems. Zwitterionic materials have been widely studied and evolved in several membrane Zwitterionicseparation materials processes have been and widely explores studied their and advantages. evolved in This several review membrane summarizes how the separation processesintroduction and explores of zwitterionic their advantages. moieties This can improve review summarizes the antifouling how characteristicsthe in‐ of FO troduction of zwitterionicmembrane without moieties deterioration can improve ofthe the antifouling membrane characteristics performance. of FO mem‐ brane without deterioration of the membrane performance. 2. Classification of Fouling Types in FO 2. Classification2.1. of Organic Fouling Fouling Types in FO 2.1. Organic FoulingOrganic fouling plays a vital role in deteriorating the membrane performance during FO [20,39,55]. Humic acid, polysaccharides, colloidal particles, protein, lipid, alginate, folic Organic fouling plays a vital role in deteriorating the membrane performance during acid are some common organic foulants reported in the literature. Solute agglomeration and FO [20,39,55]. Humic acid, polysaccharides, colloidal particles, protein, lipid, alginate, fo‐ subsequent pore blocking would result in the primary fouling of the membrane. This led to lic acid are some common organic foulants reported in the literature. Solute agglomera‐ cake formation and influence in effective surface area, surface roughness and lipophilicity tion and subsequent pore blocking would result in the primary fouling of the membrane. of the membrane surface [56,57]. This modification of the membrane surface eventually This led to cake formation and influence in effective surface area, surface roughness and aggravates the organic fouling. Due to the salt reverse flux from DS, the salt ions can be lipophilicity of the membrane surface [56,57]. This modification of the membrane surface trapped into the fouling layer. This would lead to a severe drop in water permeability eventually aggravates the organic fouling. Due to the salt reverse flux from DS, the salt and modify the hydrodynamic conditions. At extensive fouling, the phenomena highly ions can be trapped into the fouling layer. This would lead to a severe drop in water per‐ influenced by the interaction between the foulant molecules and the fouling layer generated meability and asmodify a barrier the hydrodynamic on the active surface conditions. of the membrane.At extensive Divalent fouling, cations the phenomena like Ca2+ or Mg2+ were highly influencedalso by found the tointeraction aggravate between organic fouling,the foulant specially molecules in the and presence the fouling of humic layer acid by bridging the -OH and -COOH group of the same [58]. Hydrophilic naturals were reported to have higher fouling capacity compared to hydrophilic acids, while transliphic acid has higher fouling capacity than hydrophilically charged ones, whereas the hydrophilic acid has more

fouling capacity than transliphic acid. Hence, the adhesion tendency of polysaccharide was Polymers 2021, 13, 583 4 of 18

reported to be three times greater than that of humic acid. The permeate flux was reduced because of adsorption of organic foulant, which can be expressed as follows:

− ∆π J = (1) η R

where J is the water flux, ∆π is the osmotic pressure difference between feed side and permeate side, η is the viscosity coefficient of the feed solution and R is the total resistance for organic fouling. R is a function of different factors like the intrinsic resistance of the membrane in the presence of pure water (Ri); resistance due to concentration polarization (RCP); resistance due to cake formation (RC); resistance due to pore clogging (RP) and resistance due to organic foulant adsorption (Ra). Assuming adsorption of organic foulant is the major phenomena, the water permeability can be expressed as:

1 A = (2) Ri + Ra The number of interaction sites on the membrane surface plays a vital role in deter- mining the initial rate of adsorption. If we consider the interaction sites are homogeneously distributed on the membrane surface and there is no influence on neighboring sites of interaction; then: ri = ∑ kiθi (3) i

where, ri is the initial rate of interaction between organic matter and the membrane surface. ki is the rate constant of the ith component and θi is the surface coverage. This model is known as the Langmuir model. If C is the concentration of the organic and KL is the Langmuir adsorption coefficient, then θ can be expressed as:

θ = KL C (4)

The declination in water flux would be linearly proportional to the amount of organic substance adsorbed on the membrane surface:

J0 − J = kL(C0 − C)V (5)

The kL is proportionality constant and V is the volume of permeate. The concentration of organic adsorbed on the membrane surface can be expressed as:

J0 − J C = C0 − (6) kLV   J0 − J θ = KL C0 − (7) kLV   dJ J0 − J r = = kKL C0 − (8) dt kLV

Integrating after putting the boundary condition; J = J0, when t = 0, the equation becomes:  kKL  k V t J = J0 + kLVC0 e L − 1 (9)

The natural organic matter can broadly be divided into two groups depending on their origin as follows: (1) Autochthonous: They are obtained from extracellular macromolecules from microor- ganism and carbon fixation by algae and aquatic plants. (2) Allochthones: They are obtained from the decayed parts of plants and animals. Polymers 2021, 13, 583 5 of 19

The natural organic matter can broadly be divided into two groups depending on their origin as follows: Polymers 2021, 13, 583 (1) Autochthonous: They are obtained from extracellular macromolecules5 of 18 from micro‐ organism and carbon fixation by algae and aquatic plants. (2) Allochthones: They are obtained from the decayed parts of plants and animals. The structuresThe of some structures common of some organic common foulants organic are foulants shown in are Figure shown2 . inOrganic Figure 2. Organic fou‐ foulants with largelants size/molecular with large size/molecular weight are reported weight toare cause reported reversible to cause fouling, reversible whereas fouling, whereas organic foulantsorganic of small foulants molecular of small weight molecular lead to weight irreversible lead foulingto irreversible and are fouling difficult and to are difficult to remove from theremove membrane from the surface. membrane surface.

Figure 2. SimplifiedFigure 2. chemical Simplified structures chemical for structures some of the for organic some of foulants. the organic foulants. 2.2. Inorganic Fouling 2.2. Inorganic Fouling Inorganic fouling occurs when the solubility of sparingly soluble inorganic materials Inorganic fouling occurs when the solubility of sparingly soluble inorganic materials (SiO2, CaCO3, CaSO4, MgSO4, BaSO4, etc.) present in the feed solution exceed their (SiO2, CaCO3, CaSO4, MgSO4, BaSO4, etc.) present in the feed solution exceed their solu‐ solubility limit near the membrane surface [59–62]. The reverse diffusion of bivalent salt bility limit near the membrane surface [59–62]. The reverse diffusion of bivalent salt ions ions from the draw solution was also reported to enhance the cake formation by bridging from the draw solution was also reported to enhance the cake formation by bridging and and interacting with other substances present in the feed solution. Out of the different interacting with other substances present in the feed solution. Out of the different foulants foulants responsible for inorganic scaling, silica—with a ~120 mg·L−1 solubility limit—is responsible for inorganic scaling, silica—with a ~120 mg∙L−1 solubility limit—is one of the one of the most common materials. Beyond this concentration, severe inorganic scaling most common materials. Beyond this concentration, severe inorganic scaling is reported. is reported. This silica deposition ultimately leads to the formation of silica gel films on This silica deposition ultimately leads to the formation of silica gel films on the membrane the membrane surface because of polymerization resulting in deterioration of membrane surface because of polymerization resulting in deterioration of membrane performance. performance. The nature of the membrane material was also found to significantly influence the inorganic scalingThe nature in FO of process. the membrane The stronger material adhesion was also force found between to significantly silica gel andinfluence the inor‐ the polyacetateganic membrane scaling surfacein FO process. resulted The in stronger stronger adhesion inorganic force scaling between compared silica gel to and the poly‐ that observed onacetate a cellulose membrane acetate surface membrane resulted surface in stronger [59]. A inorganic comparative scaling evaluation compared to that ob‐ of inorganic scalingserved on on thin a cellulose film composite acetate membrane surface having multiple[59]. A comparative -COOH groups evaluation of inor‐ vis-a-vis celluloseganic acetate scaling membrane on thin film was composite reported. Themembrane dipolar having -COOH multiple functional ‐COOH groups groups vis‐a‐vis resulted in dipolarcellulose interactions acetate membrane leading to was silica reported. deposition The on dipolar membrane ‐COOH surface. functional In the groups resulted next step, monosilisicin dipolar acid interactions started interacting leading to resultingsilica deposition in -Si-O- on bond membrane and eventually surface. In the next step, leading to polymerization,monosilisic acid resulting started to interacting formation resulting of silica gel. in ‐Si Severe‐O‐ bond gypsum and scalingeventually leading to was reported onpolymerization, polyamide membrane resulting compared to formation to cellulose of silica gel. acetate Severe (CA) gypsum and cellulose scaling was reported triacetate (CTA)on membrane polyamide materials membrane in FO compared operation to [59 cellulose]. acetate (CA) and cellulose triacetate (CTA) membrane materials in FO operation [59]. 2.3. Biofouling Biofouling in FO is the most complicated fouling process and is the most difficult to control. Biofouling is the deposition of a microbial community on a membrane surface followed by the formation of biofilms [63–66]. This is a multicellular architecture of live and dead cells in a self-produced gel like extracellular polymeric substances mainly containing proteins and polysaccharides. Because of the stronger adhesion, simple shear force, and physical cleaning, even the use of oxidizing substances may not be effective for the recovery of FO membranes once it undergoes extensive biofouling. This biofouling can be explained as a combination of two-step phenomena: Polymers 2021, 13, 583 6 of 18

(1) The transportation of bacterial cells near the membrane-feed solution interface and attachment of bacterial cells on the membrane surface. This step is reversible in nature and is mainly governed by van der Waal’s forces. (2) In the subsequent step, there is a formation of a bacterial colony leading to a stronger interaction with membrane materials, formation of biofilms, and eventually modifica- tion of the membrane surface properties. This step is irreversible in nature. FO biofilms are more open structures compared to those in RO mode. Most of the bacterial cell walls possess plenty of phosphoryls and carboxylic acid functional groups. As a result, the bacterial cell wall is mostly negatively charged. Therefore, a membrane surface with negative zeta potential is the ideal choice to avoid bacterial attachment. Additionally, a heterogeneous polyamide FO membrane surface was reported to be more susceptible to biofouling compared to cellulose acetate due to its hydrophobicity [67]. Biofouling can also lead to pore blocking and hence initiate other types of fouling, like inorganic scaling. Chemical cleaning with chlorine is one of the unique methods for the addressing biofouling, which can further be improved by high crossflow velocity.

2.4. Colloidal Fouling Colloidal particles are intermediate in size. They are neither truly dissolved solid nor suspended solids. They are mainly contributed by clay, corrosion products, silica, etc. [68–70]. This colloidal fouling could lead to an increase in cake-enhanced osmotic pressure resulting in reduction of driving force and the permeate flux. The colloidal fouling can affect the FO performance mainly in two ways: (1) Cake layer hydraulic resistance (2) Cake enhanced osmotic pressure (CEOP) CEOP (∆π*) can be expressed as follows:

 J  ∆π∗ = 2RTΦR C exp 0 = 2RTΦC CP∗ (10) o b k∗ b

where R is the gas constant, T is temperature (K), Φ indicates the molar osmotic coefficient, * Ro indicates the salt rejection, Cb indicates the bulk salt concentration, CP indicates the * cake-enhanced concentration polarization, J0 indicates the initial flux, k indicates the cake- hindered mass transfer coefficient. Initially, cake layer formation and cake layer hydraulic resistance play a pivotal role in colloidal fouling. However, with time, CEOP becomes the major contributing factor towards hydraulic fouling.

3. Zwitterionic Membrane To date, several strategies have been used to functionalize surfaces/membranes. Zwitterionic functionalized membrane modification could typically be divided into four methodologies: in-situ, second interfacial polymerization, coating, and atom transfer radical polymerization ATRP.

3.1. In-Situ Additives are blended in a casting solution or an interfacial polymerization pre- cursor solution, which is an easy method to fabricate a functionalized FO membrane. Thus, one single step “in-situ” modification has attracted researchers’ and membrane manufacturers’ attention [71]. Ismail’s group [72] synthesized a zwitterionic polymer, poly[3-(N-2-methacryloylxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS) and blended it with polyethersulfone (PES) to fabricate a substrate to form a TFC-selective layer following interfacial polymerization. With a higher PMAPS content, the flux could increase from 12.54 to 15.79 L·m−2h−1 (1% PMAPS) using 2 M NaCl draw solution due to the enhancement of hydrophilicity. However, overloaded PMAPS led to a significant reverse solute flux jumping from 3.56 gMH (0% PMAPS) to 24.52 gMH (5% PMAPS) be- cause of bigger pore size support membrane that causes the PA layer to not fully cover Polymers 2021, 13, 583 7 of 18

the membrane surface. To test the antifouling behavior, an oily wastewater treatment was performed in the active layer facing the draw solution (AL-DS) mode zwitterionic support layer facing to the feed stream. Owing to the dense selective layer, the oil rejection was maintained at 99%. Furthermore, a four cycles long-term test along with a high oil concentration were used to evaluate the antifouling performance. After increasing the concentration, the flux decline was severe for the control membrane. The normalized water flux dropped at 51.2% for 10,000 ppm oil, which was higher than 1% PMAPS-PES ~34.8%. This was because the SO3 forms a hydration layer with water molecules, which prevent oil droplet adhesion onto the membrane support. Chiao et al. [13] synthesized a zwitterionic diamine monomer, N-aminoethyl piperazinepropane sulfonate (AEPPS), which could mix in the interfacial polymerization precursor aqueous solution and react with the organic phase precursor TMC to form a cross-linked polyamide layer on the membrane. The main purpose to this resulting membrane was to augment the antifouling ability and enhance the membrane performance. The water flux was gradually increased as more AEPPS was involved into the reaction, from ~10 L·m−2h−1 (0% AEPPS) to ~17 L·m−2h−1 (50% AEPPS). The 30% had the lowest specific slat flux ~0.4 g/L. Because of the tight hydration layer, improved antifouling properties were expected. Thus, the static and dynamic fouling were used to challenge and prove it. The model protein foulants BSA and lysozyme solution were used to determine the adsorption ability on the membrane surface. At pH 7, BSA exists as negatively charge protein, whereas lysozyme exists as positively charged protein (their isoelectric point is as follow: BSA= pH 4.7, and lysozyme= pH 11.0) [47]. With more AEPPS participating the reaction, the surface adsorption capacity was reduced either using BSA or lysozyme solution. In the dynamic fouling test, 800 ppm sodium alginate was used to be a model organic foulant which can form a gel layer and decrease the water permeability. 50% AEPPS membrane could maintain normalized flux more than 75% after 700 min of FO mode operation, which was higher than 0% AEPPS membrane 60%. This resulting zwitterionic membrane not only enhanced the water flux, but also the static and dynamic antifouling ability due to the surface hydrophilicity formed by the zwitterionic tight hydration layer. However, the selectivity structure or crosslinking degree is changing. There is a possibility to lose the selectivity ability and increase the solute reverse flux even the permeability could be enhanced. The amount of additive must be added carefully, as it plays an important role to in-situ modification [14]. Figure3 schematically presents the in-situ inclusion of zwitterionic moieties in a FO membrane.

3.2. Second Interfacial Polymerization (SIP) Second interfacial polymerization is regarded as an in-situ modification technique [73], however, the SIP route does not obviously affect the structure of the PA selectivity layer. The PA layer is functionalized by additives with a covalent bond to cross-link with the typical PA structure. Thus, it is discussed independently and separately from in-situ modification. Chiao et al. [14] synthesized the zwitterionic monomer AEPPS from 1-(2- aminoethyl)piperazine (AEP) and 1,3-propanesultone via a ring-opening reaction. The amine group on AEPPS reacted with the chloride acid group on the TMC after generating a typical selectivity structure PA. The resulting functionalized PA layer (PAZ) enhanced the hydrophilicity significantly from ~80◦ to ~15◦, because of the tightly-bound water layer around the zwitterionic moieties. Meanwhile, the super-oleophobicity was found for PAZ (~160◦) in the underwater oil contact angle test. In general, the superoleophobic surface has the value above 150◦ [74]. From the morphology, the mitigated nodule structure and smoother surface was found after SIP, which was beneficial for the antifouling property. The water flux was improved from 11.48 to 18.91 L·m−2h−1 using 1 M NaCl solution as a draw solution in FO mode. Through the SIP, the structure parameter (S) was reduced about 43% compared to the controlled membrane because of the superhydrophilicity of the zwit- terionic moieties. Meanwhile, the antibacterial property of the zwitterionic functionalized membrane significantly improved as expected [75], where the amount of attached E. coli was reduced from 4810 cell/mm2 to 352 cell/mm2. Besides, the protein foulants BSA, and Polymers 2021, 13, 583 8 of 18

lysozyme was used to investigate the performance. At pH 7.4, the protein surface charge of BSA and lysozyme were −22 and +3.5 mV; consequently, the positive and negative charge could be a good model foulant to challenge the zwitterionic surface. Modified membrane PAZ demonstrated an excellent antifouling behavior attributed to the surface charge and roughness. Apart from a static test, a dynamic fouling test using simulated city wastewater, which is a common model solution to simulate the polysaccharides present in real wastewater, was performed too. The normalized flux of the modified membrane could be maintained above 90% before achieving 50% recovery. The real produced water was filtered with a PAZ membrane. In the commercial aspect, reduction of wastewater discharge is an enormous challenge when using the horizontal drilling and hydraulic frac- turing technique. It consists of oil and grease, salt, surfactants and so on; thus, regulators strictly define the allowed discharge locations. Chen et al. designed a zwitterionic amine monomer, (1-(3-aminopropyl) imidazole) propane sulfonate (APIS) and grew it on a PA layer. The surface hydrophilicity decreased significantly from ~70◦ (PA-PES) to ~40◦; mean- Polymers 2021, 13, 583 while, 101% higher water flux than the unmodified membrane [76]. Figure8 of 194 schematically

presents the inclusion of zwitterionic moieties by 2nd interfacial polymerization.

Figure 3. TheFigure schematic 3. The schematic diagram diagram for in-situ for inclusion in‐situ inclusion of zwitterionic of zwitterionic moieties moieties in FO in membrane FO membrane (reproduced from Chiao et al. [(reproduced13]). from Chiao et al. [13]).

3.2. Second Interfacial3.3. Coating Polymerization (SIP) Second interfacialZwitterion polymerization functionalized is regarded carbon as nanotubes an in‐situ (Z-CNTs)modification coated technique onto a commercial ® [73], however,Aquaporin the SIP route Inside does notFO obviously thin film affect composite the structure (TFC) membrane of the PA selectivity have been reported by layer. The PA Zoulayer et is al. functionalized [77]. They obtained by additives BSF mitigation with a covalent through bond zwitterion-induced to cross‐link with repulsive electro- static interaction along with the carbon nanotube-induced steric interaction. The presence the typical PA structure. Thus, it is discussed independently and separately from in‐situ of extended repulsive interaction in presence of low ionic strength liquid, the coating of modification. Chiao et al. [14] synthesized the zwitterionic monomer AEPPS from 1‐(2‐ zwitterion-functionalized CNT on the active side of the membrane was found to provide aminoethyl)piperazine (AEP) and 1,3‐propanesultone via a ring‐opening reaction. The better mitigation of reverse salt flow in active layer feed facing mode. They have reported amine group on AEPPS reacted with the chloride acid group on the TMC after generating a reduction in specific reverse salt flow of 84% for using NH H PO as draw solution, 75% a typical selectivity structure PA. The resulting functionalized PA layer (PAZ)4 2 enhanced4 for (NH ) HPO as draw solution, 71% for NH Cl as draw solution using an optimum the hydrophilicity significantly4 2 4 from ~80° to ~15°, because of the4 tightly‐bound water layer coating density of 0.97 g m−2. After 12 days operation of actual wastewater, only marginal around the zwitterionic moieties. Meanwhile, the super‐oleophobicity was found for PAZ (~160°) in the underwater oil contact angle test. In general, the superoleophobic surface has the value above 150° [74]. From the morphology, the mitigated nodule structure and smoother surface was found after SIP, which was beneficial for the antifouling property. The water flux was improved from 11.48 to 18.91 L∙m−2h−1 using 1 M NaCl solution as a draw solution in FO mode. Through the SIP, the structure parameter (S) was reduced about 43% compared to the controlled membrane because of the superhydrophilicity of the zwitterionic moieties. Meanwhile, the antibacterial property of the zwitterionic func‐ tionalized membrane significantly improved as expected [75], where the amount of at‐ tached E. coli was reduced from 4810 cell/mm2 to 352 cell/mm2. Besides, the protein fou‐ lants BSA, and lysozyme was used to investigate the performance. At pH 7.4, the protein surface charge of BSA and lysozyme were −22 and +3.5 mV; consequently, the positive and negative charge could be a good model foulant to challenge the zwitterionic surface. Modified membrane PAZ demonstrated an excellent antifouling behavior attributed to the surface charge and roughness. Apart from a static test, a dynamic fouling test using simulated city wastewater, which is a common model solution to simulate the polysac‐ charides present in real wastewater, was performed too. The normalized flux of the mod‐ ified membrane could be maintained above 90% before achieving 50% recovery. The real

Polymers 2021, 13, 583 9 of 19

produced water was filtered with a PAZ membrane. In the commercial aspect, reduction of wastewater discharge is an enormous challenge when using the horizontal drilling and hydraulic fracturing technique. It consists of oil and grease, salt, surfactants and so on; Polymers 2021, 13, 583 thus, regulators strictly define the allowed discharge locations. Chen et al. designed a9 of 18 zwitterionic amine monomer, (1‐(3‐aminopropyl) imidazole) propane sulfonate (APIS) and grew it on a PA layer. The surface hydrophilicity decreased significantly from ~70° (PA‐PES)reduction to ~40°; in meanwhile, water flux (~15% 101% higher reduction) water was flux reported than the compared unmodified to membrane[76]. that of virgin mem- Figurebrane 4 schematically (~55% reduction). presents Simple the physical inclusion flushing of zwitterionic was found moieties to be effective by 2nd for interfacial the removal polymerization.of any foulants adsorbed on the membrane surface.

FigureFigure 4. Schematics 4. Schematics of 2nd of interfacial 2nd interfacial polymerization polymerization for inclusion for inclusion of zwitterionic of zwitterionic moieties moieties on FO on FO membrane (reproduced from Chen et al. ). membrane (reproduced from Chen et[76] al. [76]). Polymers 2021, 13, 583 10 of 19

3.3. CoatingNeguyen et al. [78] hatve reported the surface modification of commercially available flatZwitterion sheet cellulose functionalized triacetate carbon FO membranenanotubes (Z by‐CNTs) coating coated of a zwitterion onto a commercial (polyamino Aq‐ acid 3-(3,4-dihydroxyphenyl)-L-alanine® (L-DOPA)) on the porous side of the membrane, to uaporin(3,4‐dihydroxyphenyl) Inside FO thin film‐L‐alanine composite (L‐DOPA)) (TFC) membraneon the porous have side been of thereported membrane, by Zou to et im ‐ al. prove[77].improve They the antifouling theobtained antifouling BSF characteristics mitigation characteristics throughof membrane of membrane zwitterion in pressure in‐ pressureinduced retarded retardedrepulsive osmosis osmosis electrostatic mode. mode. As As interactionindicatedindicated along earlier, earlier, with the the the coating coating carbon of nanotube zwitterions‐induced onon membrane membrane steric interaction. surface surface would would The naturally presence naturally enhance of en ‐ the hydrophilicity of the membrane surface and the repulsive electrostatic interaction extendedhance the repulsive hydrophilicity interaction of thein presence membrane of low surface ionic and strength the repulsive liquid, the electrostatic coating of zwitinterac‐ ‐ provided by the cationic as well as anionic moieties of zwitterions lead to the reduction in teriontion‐ functionalizedprovided by the CNT cationic on the as active well asside anionic of the membranemoieties of waszwitterions found to lead provide to the better reduc ‐ fouling propensity. The L-DOPA-coated membranes showed 30% less fouling probability mitigationtion in fouling of reverse propensity. salt flow Thein active L‐DOPA layer‐ coatedfeed facing membranes mode. They showed have 30 reported % less foulinga re‐ compared to a virgin membrane after 12 h of FO operation in PRO mode using alginic ductionprobability in specific compared reverse to asalt virgin flow membrane of 84 % for after using 12 hNH of 4FOH2PO operation4 as draw in PROsolution, mode 75 using % acid sodium salt and CaCl2 as feed solution. Almost 90% recovery of the L-DOPA-coated foralginic (NH4) 2acidHPO sodium4 as draw salt solution, and CaCl 712 as % feed for solution.NH4Cl as Almost draw solution90 % recovery using ofan the optimum L‐DOPA ‐ membrane was reported−2 by simple hydraulic water treatment. Figure5 schematically coatingcoated density membrane of 0.97 was g m reported. After 12by dayssimple operation hydraulic of actualwater treatment.wastewater, Figure only marginal5 schemati ‐ present coating of zwitterions on FO membranes. reductioncally present in water coating flux (~of 15zwitterions % reduction) on FO was membranes. reported compared to that of virgin mem‐ brane (~55 % reduction). Simple physical flushing was found to be effective for the re‐ moval of any foulants adsorbed on the membrane surface. Neguyen et al. [78] ve reported the surface modification of commercially available flat sheet cellulose triacetate FO membrane by coating of a zwitterion (polyamino acid 3‐

FigureFigure 5. The 5. The schematic schematic of offormation formation of ofzwitterionic zwitterionic polymer polymer coated coated FO FO membrane membrane[78] [78. ].

3.4. Atom Transfer Radical Polymerization (ATRP) ATRP is a technique for control the grafting of zwitterions on the membrane surface. The first step is to immobilize the initiator, the site for polymer growth. The second step involves the controlled polymerization of the zwitterionic monomer, mainly utilizing the equilibrium of Cu2+/Cu1+ salt (Figure 6) [79–81]. The duration of the initiator immobiliza‐ tion step influences the grafting density, while the length of the zwitterionic polymeric chain was influenced by the duration of the 2nd step of ATRP. Liu et al. [82] reported a comprehensive investigation in understanding the efficacy of the poly(sulfobetaine meth‐ acrylate) grafted thin‐film composite membrane vis‐a‐vis a dense layer of hydrophilic sil‐ ica nanoparticle impregnated thin‐film composite FO membrane towards antifouling characteristics. The nanoparticle‐impregnated membrane was found to be ineffective in shielding the surface carboxylic acid groups resulting in an electrostatic attractive interac‐ tion with protein molecules and Ca2+ ion present as foulant. However, the presence of a polymeric zwitterionic chain on the membrane surface not only enhanced the surface hy‐ drophilicity, but also improved the hydration layer of zwitterionic polymeric brush pro‐ vided a large physical as well as energetic barrier to shield the carboxylic acid functional group. Thus, neither protein molecules nor inorganic/organic moieties can be adsorbed on the membrane surface. The microorganism adsorption test was performed using E coli. A significant number of E. coli cells (1.44 ± 0.30 × 105 cells/cm2) was found to grow in pres‐ ence of virgin pristine thin film composite membrane. However, zwitterionic polymer grafted TFC membrane led to a reduction in 96 % of E. coli cell counts, revealing the anti‐ microbial characteristics of the same.

Polymers 2021, 13, 583 10 of 18

3.4. Atom Transfer Radical Polymerization (ATRP) ATRP is a technique for control the grafting of zwitterions on the membrane surface. The first step is to immobilize the initiator, the site for polymer growth. The second step involves the controlled polymerization of the zwitterionic monomer, mainly utilizing the equilibrium of Cu2+/Cu1+ salt (Figure6)[ 79–81]. The duration of the initiator immobiliza- tion step influences the grafting density, while the length of the zwitterionic polymeric chain was influenced by the duration of the 2nd step of ATRP. Liu et al. [82] reported a comprehensive investigation in understanding the efficacy of the poly(sulfobetaine methacrylate) grafted thin-film composite membrane vis-a-vis a dense layer of hydrophilic silica nanoparticle impregnated thin-film composite FO membrane towards antifouling characteristics. The nanoparticle-impregnated membrane was found to be ineffective in shielding the surface carboxylic acid groups resulting in an electrostatic attractive inter- action with protein molecules and Ca2+ ion present as foulant. However, the presence of a polymeric zwitterionic chain on the membrane surface not only enhanced the surface hydrophilicity, but also improved the hydration layer of zwitterionic polymeric brush pro- vided a large physical as well as energetic barrier to shield the carboxylic acid functional group. Thus, neither protein molecules nor inorganic/organic moieties can be adsorbed on the membrane surface. The microorganism adsorption test was performed using E coli. A significant number of E. coli cells (1.44 ± 0.30 × 105 cells/cm2) was found to grow in presence of virgin pristine thin film composite membrane. However, zwitterionic poly- Polymers 2021, 13, 583 11 of 19 mer grafted TFC membrane led to a reduction in 96% of E. coli cell counts, revealing the antimicrobial characteristics of the same.

Figure 6. The schematic presentation for ATRP to graft zwitterionic brush. Figure 6. The schematic presentation for ATRP to graft zwitterionic brush. Poly [2-(methacryloyloxy)-ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (PSBMA)Poly was[2‐(methacryloyloxy) grafted on a commercial‐ethyl] dimethyl pristine FO‐(3‐ membrane.sulfopropyl) It ammonium was found tohydroxide produce a(PSBMA) zwitterionic was grafted TFC membrane on a commercial having enhancedpristine FO hydrophilicity, membrane. It reducedwas found surface to produce charge a andzwitterionic surface TFC roughness membrane [83]. having PSBMA-grafted enhanced hydrophilicity, TFC membrane reduced was reported surface charge to have and a narrowersurface roughness adhesion [83]. force PSBMA distribution,‐grafted ca, TFC−0.07 membrane± 0.15 mN/m, was reported revealing to have a 92% a reduction narrower comparedadhesion force to virgin distribution, membrane. ca, − The0.07 effective ± 0.15 mN/m, shielding revealing of carboxylic a 92% acidreduction groups compared resulted to virgin membrane. The effective shielding of carboxylic acid groups resulted in the pre‐ vention of Ca2+‐induced fouling. Additionally, the swelling of the zwitterionic polymeric chain resulted in a further reduction of the adhesive force because of the increase of dis‐ tance between the solid‐water interface and the membrane surface. The protein adsorp‐ tion was monitored using a FITC‐BSA model by fluorescence spectroscopy. The highly intense fluorescent membrane surface for virgin membrane was attributed to the BSA ad‐ sorption, which led to almost no fluorescent membrane surface after zwitterionic poly‐ meric brush grafting. This indicated the antiprotein adsorption characteristics of the mem‐ brane due to the presence of zwitterions. The zwitterionic modification was claimed to result in an almost 90% reduction in colony forming units (CFUs) against E coli bacteria. Static and dynamic fouling experiments confirmed that the presence of zwitterions on the membrane surface not only improved the hydrophilicity of the surface resulting in an enhancement in membrane permeability, but also extensively improved the antifouling characteristics of the membrane towards inorganic scaling, protein adsorption, and even bacterial/biofilm growth. Zhang et al. [84] investigated different cleaning approaches for FO membranes hav‐ ing zwitterionic poly‐(sulfobetaine methacrylate) brushes prepared by ATRP subjected to crude oil and anionic polyacrylamide as model fouling solution for treatment of polymer flooding‐produced water. They reported a significant enhancement in irreversible fouling with increasing relative concentration of crude oil in the feed solution. Five different clean‐ ing protocols have been adopted as follows: (1) Simple hydraulic washing: Feed: DI water; Draw solution: DI water; pH 7 (2) Osmotic backwashing: Feed: 2 M NaCl, Draw solution: DI water; pH 7

Polymers 2021, 13, 583 11 of 18

in the prevention of Ca2+-induced fouling. Additionally, the swelling of the zwitterionic polymeric chain resulted in a further reduction of the adhesive force because of the increase of distance between the solid-water interface and the membrane surface. The protein adsorption was monitored using a FITC-BSA model by fluorescence spectroscopy. The highly intense fluorescent membrane surface for virgin membrane was attributed to the BSA adsorption, which led to almost no fluorescent membrane surface after zwitterionic polymeric brush grafting. This indicated the antiprotein adsorption characteristics of the membrane due to the presence of zwitterions. The zwitterionic modification was claimed to result in an almost 90% reduction in colony forming units (CFUs) against E coli bacteria. Static and dynamic fouling experiments confirmed that the presence of zwitterions on the membrane surface not only improved the hydrophilicity of the surface resulting in an enhancement in membrane permeability, but also extensively improved the antifouling characteristics of the membrane towards inorganic scaling, protein adsorption, and even bacterial/biofilm growth. Zhang et al. [84] investigated different cleaning approaches for FO membranes having zwitterionic poly-(sulfobetaine methacrylate) brushes prepared by ATRP subjected to crude oil and anionic polyacrylamide as model fouling solution for treatment of polymer flooding-produced water. They reported a significant enhancement in irreversible fouling with increasing relative concentration of crude oil in the feed solution. Five different cleaning protocols have been adopted as follows: (1) Simple hydraulic washing: Feed: DI water; Draw solution: DI water; pH 7 (2) Osmotic backwashing: Feed: 2 M NaCl, Draw solution: DI water; pH 7 (3) Acid cleaning: Feed: 0.1% Citric acid; Draw solution: DI water; pH 3 (4) Alkaline cleaning: Feed: 0.1% NaOH, Draw solution: DI water; pH 12 (5) Surfactant cleaning: Feed: 0.1% SDBS solution; Draw solution: DI water; pH 9 Simple hydraulic washing was found to be an inefficient procedure, while an excellent cleaning efficiency and chemical stability was reported for poly(sulfobetaine methacrylate)- grafted FO membranes towards the surfactant cleaning agents. However, poor chem- ical stability was reported for high pH alkali cleaning agents. A gradual formation of irreversible fouling on PSBMA grafted membrane was reported and attributed to the insufficient capability of the DI water for removal of the accumulated oil foulants from the membrane surface. The synergistic effect of the NaCl solubilization and water back- flushing was found to be responsible for the increase in cleaning efficiency of the foulants accumulated on the membrane surface. The SDBS cleaning protocol was demonstrated to be the most efficient protocol to remove foulants from the surface as this surfactant formed a macromolecular micellar agglomeration to dissolve oil, fat protein and other lyophilic foulants in water.

4. Draw Solution The draw solution in FO exhibits a very important role in improving the water permeability and the back-diffusion of the components of the draw solution. Though conventionally, inorganic salts solution (e.g., NaCl, MgSO4. etc.) was used as a draw solution in FO process, a vast amount of research has been carried out on the nature of draw solutions and their effect on FO performance. Application of organic molecules as draw solution was reported to induce advantageous properties. The size being larger compared to inorganic cations, the rate of back diffusion is slower [85]. Reports were also available on using highly volatile diethyl ether as a draw solution to facilitate the recovery of the permeate. Polyelectrolytes have also been exploited as a draw solution to reduce the reverse solution flux. However, only a marginal improvement in FO performance was noticed compared to the presence of organic molecules as draw solution. The formation of a cross-linking network with water molecules, and the stimuli-imposed dehydration characteristic of hydrogel have been exploited to use hydrogels in the draw solution. Reduction in osmotic pressure because of the Gibb’s-Donnan effect limits the application of Polymers 2021, 13, 583 12 of 18

hydrogel. Using stimuli responsive polymer (mainly temperature, chemical environment, pH, etc.) as draw solution has also drawn the interest of researchers. Ju and Kang have reported the application of zwitterionic polysulfobetaine homopoly- mers as draw solution in FO performance [85]. The upper critical solution temperature for polysulfobetaine- water was reported to be 40 ◦C. This implies that if the solution temperature is less than that of 40 ◦C, the ion-ion interaction between the zwitterionic moieties is stronger than that of zwitterion-water interaction resulting in zwitterionic ag- glomeration and phasing out from aqueous medium, whereas the reverse is true for the solution temperature more than 40 ◦C, leading to homogeneous zwitterion-water solution. Thus, after FO operation, the recovery of water from the polymeric zwitterionic aqueous solution was achieved by the temperature response of the polymer. As the concentration of the polymeric zwitterionic brush was enhanced from 5 wt% to 10 to 15 to 20 wt%; the water flux values followed the order, 0.92, 1.39, 2.30, 3.22 L·m−2h−1, respectively. The reverse solute flux was reported to increase with the increase in draw solution concentration as follows: 0.27 gMH at 5 wt%, 0.29 gMH at 10 wt%, 0.35 gMH at 15 wt%, and 0.36 gMH at 20 wt%. Their investigation provided a new dimension in research on draw solute. Lutchmia et al. [86] reported a comparative evaluation on the application of differ- ent zwitterions (glycine, L-proline, L-valine, L-glutamine and glycine betaine) as draw solution in FO process vis-a-vis aqueous NaCl solution, the conventional draw solution in wastewater reclamation. The investigation revealed that glycine, L-proline and glycine betaine exhibited comparable water fluxes to the conventional draw solution, NaCl and the reported water flux was 5 L·m−2h−1. However, a significantly low solute loss was reported by using these zwitterions in draw solution (glycine: 2.13 g/m2h; L-proline: 1.377 g/m2h; glycine betaine: 0.967 g/m2h; and NaCl: 3.26 g/m2h). The physicochemical parameters of zwitterions as draw solution, which significantly influence the FO operation are as follows: (1) Osmotic pressure (2) Charge (3) Polarity (4) Molecular weight

Osmotic Pressure: The diluted internal concentration polarization for conventional NaCl solutions was reported to be less than that of zwitterions. Zwitterionic glycine showed severe diluted internal concentration polarization compared to proline. The diffusivity and the viscosity of the draw solution significantly influenced the diluted internal concentration polariza- tion. Highly soluble draw solutes were found to enhance the water permeability with enhancement in osmotic pressure difference.

Charge: Zwitterions possess dipoles because of the separation of the positive and negative charge in the same moiety. In the case of amino acids, the zwitterionic form and its concentration depend on the pH of the medium as it is significantly influenced by pKa of the materials. According to the Henderson-Hasselbalch equation, the pKa and pH are quantitatively related as follows:

 [Dissociated form]  pH = pKa + Log (11) 10 [Undissociated form]

Polarity: The partitioning of the zwitterions in water vs the membrane materials played a significant role in determining the (Js/Jv) flux ratio. As the hydrophobicity of the zwitterion decrease, the Js/Jv value was also reported to decrease. The polarity of the zwitterions was reported to influence their rejection. Polymers 2021, 13, 583 13 of 18

Molecular Weight: Because of steric hindrance, which is also called the sieving effect, the molecular weight of the zwitterions in the draw solution highly influences the reverse solute flux as discussed earlier. The diffusion coefficient of the zwitterions through the membrane pores depends on the molecular weight of the zwitterions. The molecular weight followed the trend: glutamine > glycine betaine > NaCl resulted in the trend in Js/Jv ratio NaCl > glycine betaine > glutamine. The molecular weight followed the trend: glutamine > valine ~ proline > glycine > NaCl resulting in a diffusion coefficient trend: NaCl > glycine > Polymers 2021, 13, 583 14 of 19 proline > valine > glutamine. Figure7 shows the chemical structures for some zwitterions used as draw solutions.

FigureFigure 7. Chemical 7. Chemical structures structures of some of some of the of the zwitterions zwitterions used used as asdraw draw solution solution in inFO. FO. 5. Perspectives 5. Perspectives The application of zwitterions as a draw solution has opened up a new domain of The application of zwitterions as a draw solution has opened up a new domain of research, however, there is always a tradeoff between the osmotic pressure difference research, however, there is always a tradeoff between the osmotic pressure difference cre‐ created by the draw solute and the recovery of the permeated products from the draw ated by the draw solute and the recovery of the permeated products from the draw solu‐ solution side. The future direction of research in this topic is to effectively overcome this tion side. The future direction of research in this topic is to effectively overcome this tradeoff. Zwitterionic polymers in combination with their external stimuli response could tradeoff. Zwitterionic polymers in combination with their external stimuli response could resolve the issue. In view of that, some literature is available where temperature responsive resolve the issue. In view of that, some literature is available where temperature respon‐ polyzwitterionic moieties have been utilized [85]. The zwitterionic moieties are responsible sive polyzwitterionic moieties have been utilized [85]. The zwitterionic moieties are re‐ for creating an osmotic pressure difference between the feed and permeate side, the driving sponsibleforce for for FO. creating Subsequently, an osmotic the pressure temperature difference of the between system is the allowed feed and to permeate raise. The side, rise in thetemperature driving force results for FO. in Subsequently, a conformational the changetemperature from of an the extended system structure is allowed to to a globuleraise. Thestructure rise in temperature of the zwitterionic results polymers.in a conformational As a result, change at elevated from an temperature, extended structure the polymer to a globulephases outstructure from the of systemthe zwitterionic and hence polymers. leads to an As almost a result, complete at elevated recovery temperature, of the permeate. the polymerSimilarly, phases magnetic out from responsive the system zwitterionic and hence polymers leads to can an also almost be another complete ideal recovery choice. of The thezwitterionic permeate. Similarly, moieties willmagnetic impose responsive an osmotic zwitterionic pressure difference, polymers creatingcan also drivingbe another force idealfor choice. FO. After The the zwitterionic FO process, moieties the polymers will impose could an be osmotic separated pressure from the difference, draw solution creat‐ by ingmeans driving of force magnetic for FO. interactions. After the FO process, the polymers could be separated from the draw solutionFor the by incorporation means of magnetic of zwitterionic interactions. moieties in FO membranes, the distribution ofFor zwitterionic the incorporation moieties of on zwitterionic the membrane moieties surface in FO should membranes, be highly the homogeneous distribution of and zwitterionicthere should moieties be some on controlthe membrane of the grafting surface densityshould be and highly the length homogeneous of the polymers. and there The shouldstability be some of such control layer of on the the grafting virgin membranedensity and should the length be another of the polymers. aspect consider. The stability Instead of ofsuch considering layer on the physical virgin interaction membrane toshould attach be the another zwitterionic aspect moietiesconsider. on Instead membrane, of con the‐ sideringchemical physical interaction interaction would to exhibit attach betterthe zwitterionic stability for moieties the attachment. on membrane, the chemi‐ cal interaction would exhibit better stability for the attachment.

6. Conclusions The main applications of zwitterions in FO processes can broadly be classified into two categories: inclusion in membrane materials and inclusion in draw solutions. The presence of positive as well as negative charges on zwitterions induces an electrostatic repulsive interaction when placed on a FO membrane surface towards the foulant present in feed solution. This not only reduces the initial microbial adhesion, inorganic scaling, but also drastically reduces bacterial colony growth. The presence of zwitterions on the FO membrane surface enhances the surface hydrophilicity and surface zeta potential com‐ pared to the virgin membrane. This enhancement in surface hydrophilicity along with the formation of an aqueous layer on the FO membrane surface resists the organic molecules to have a close proximity to feed/membrane interface. This interaction leads to a drastic reduction in organic fouling on the membrane surface. Most bacterial cell walls have a

Polymers 2021, 13, 583 14 of 18

6. Conclusions The main applications of zwitterions in FO processes can broadly be classified into two categories: inclusion in membrane materials and inclusion in draw solutions. The presence of positive as well as negative charges on zwitterions induces an electrostatic repulsive interaction when placed on a FO membrane surface towards the foulant present in feed solution. This not only reduces the initial microbial adhesion, inorganic scaling, but also drastically reduces bacterial colony growth. The presence of zwitterions on the FO membrane surface enhances the surface hydrophilicity and surface zeta potential compared to the virgin membrane. This enhancement in surface hydrophilicity along with the formation of an aqueous layer on the FO membrane surface resists the organic molecules to have a close proximity to feed/membrane interface. This interaction leads to a drastic reduction in organic fouling on the membrane surface. Most bacterial cell walls have a negatively charge, hence when exposed to the anionic moieties of zwitterions, there is a repulsive electrostatic interaction, resulting in a reduction in bacterial adhesion. On the other hand, zwitterionic inclusion in draw solutions reduces the reverse solute flux compared to conventional brine solutions where inorganic salts are used. The polarity, molecular weight, and charge of zwitterions in the draw solution are the important factors controlling the reverse salt effect. Small membrane pores and large good water-soluble zwitterions are the ideal choice for FO performance.

Author Contributions: Conceptualization, writing—original draft preparation, writing—review and editing, funding acquisition, Y.-H.C.; writing—original draft preparation, writing—review and editing, A.S., S.-T.C.; writing—review and editing, M.B.M.Y.A. and T.Y.H.; funding acquisition, J.A., S.R.W., W.-S.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Membrane Science Inc. (Taiwan) through the NSF Indus- try/University Cooperative Research Center for Membrane Science, Engineering, and Technology; the National Science Foundation (IIP 1361809, 1822101, 1848682); Dana Modal Insan (DMI) MI-2019-017, Universiti Kebangsaan Malaysia; and the University of Arkansas Chancellor’s Commercialization Fund. Acknowledgments: Authors acknowledge the R&D Center for Membrane Technology at Chung Yuan Christian University, National Taiwan University of Science and Technology (NTUST), the Arkansas research Alliance, and the University of Arkansas. A. Sengupta acknowledge P.K.Pujari, Director, RC & I G and Head, Radiochemistry Division, BARC, Mumbai, India. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

A Water permeability AEP 1-(2-Aminoethyl) piperazine AEPPS N-aminoethyl piperazinepropane sulfonate Ag NPs Silver nanoparticles AL-DS Active layer facing the draw solution APIS (1-(3-Aminopropyl) imidazole) propane sulfonate ATRP Atom transfer radical polymerization BSA Bovine serum albumin CA Cellulose acetate Cb Bulk salt concentration CBMA Carboxybetaine methacrylate CEOP Cake enhanced osmotic pressure CFU Colony forming unit CP Concentration polarization CP* Cake-enhanced concentration polarization CQDs Carbon quantum dots CTA Cellulose triacetate DS Draw solution Polymers 2021, 13, 583 15 of 18

ECP External concentration polarization FO Forward osmosis FS Feed solution GO Graphene oxide HEMA 2-Hydroxyethyl methacrylate ICP Internal concentration polarization J Water flux J0 Initial flux k* Cake-hindered mass transfer coefficient ki Rate constant of the ith component KL Langmuir adsorption coefficient L-DOPA Polyamino acid 3-(3,4-dihydroxyphenyl)-L-alanine MD Membrane distillation NF Nanofiltration OEG Oligo(ethylene glycol) PBMA Phosphobetaine methacrylate PES Polyethersulfone PMAPS Poly[3-(N-2-methacryloylxyethyl-N,N-dimethyl)-ammonatopropanesulfonate] PSBMA Poly [2-(methacryloyloxy)-ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide Ra Resistance due to organic foulant adsorption RC Resistance due to cake formation RCP Resistance due to concentration polarization Ri Intrinsic resistance of the membrane in presence of pure water ri Initial rate of interaction between organic matter and the membrane surface RO Reverse osmosis Ro Salt rejection SIP Second interfacial polymerization T Temperature (K) TFC Thin film composite VOPSs Vertically oriented porous substrates Z-CNTs Zwitterion functionalized carbon nanotubes Φ Molar osmotic coefficient ∆π Osmotic pressure difference η Viscosity coefficient

References 1. Alihemati, Z.; Hashemifard, S.A.; Matsuura, T.; Ismail, A.F.; Hilal, N. Current status and challenges of fabricating thin film composite forward osmosis membrane: A comprehensive roadmap. Desalination 2020, 491, 114557. [CrossRef] 2. Zhao, S.; Zou, L.; Tang, C.Y.; Mulcahy, D. Recent developments in forward osmosis: Opportunities and challenges. J. Membr. Sci. 2012, 396, 1–21. [CrossRef] 3. Glater, J. The early history of reverse osmosis membrane development. Desalination 1998, 117, 297–309. [CrossRef] 4. Belfort, G. Membrane Filtration with Liquids: A Global Approach with Prior Successes, New Developments and Unresolved Challenges. Angew. Chem. Int. Ed. 2018, 58, 1892–1902. 5. Mulder, J.X. Basic Principles of Membrane Technology; Springer Science & Business Media: Dordrecht, The Netherlands, 2012. 6. Amy, G.; Ghaffour, N.; Li, Z.; Francis, L.; Linares, R.V.; Missimer, T.; Lattemann, S. Membrane-based seawater desalination: Present and future prospects. Desalination 2017, 401, 16–21. [CrossRef] 7. Cath, T.Y.; Childress, A.E.; Elimelech, M. Forward osmosis: Principles, applications, and recent developments. J. Membr. Sci. 2006, 281, 70–87. [CrossRef] 8. Chung, T.-S.; Zhang, S.; Wang, K.Y.; Su, J.; Ling, M.M. Forward osmosis processes: Yesterday, today and tomorrow. Desalination 2012, 287, 78–81. [CrossRef] 9. Elimelech, M.; Phillip, W.A. The future of seawater desalination: Energy, technology, and the environment. Science 2011, 333, 712–717. [CrossRef] [PubMed] 10. Wei, J.; Qiu, C.; Wang, Y.-N.; Wang, R.; Tang, C.Y. Comparison of NF-like and RO-like thin film composite osmotically-driven membranes—Implications for membrane selection and process optimization. J. Membr. Sci. 2013, 427, 460–471. [CrossRef] 11. Yao, Z.; Peng, L.E.; Guo, H.; Qing, W.; Mei, Y.; Tang, C.Y. Seawater pretreatment with an NF-like forward osmotic mem- brane: Membrane preparation, characterization and performance comparison with RO-like membranes. Desalination 2019, 470. [CrossRef] Polymers 2021, 13, 583 16 of 18

12. Chiao, Y.-H.; Sengupta, A.; Chen, S.-T.; Hung, W.-S.; Lai, J.-Y.; Upadhyaya, L.; Qian, X.; Wickramasinghe, S.R. Novel thin-film composite forward osmosis membrane using polyethylenimine and its impact on membrane performance. Sep. Sci. Technol. 2020, 55, 590–600. [CrossRef] 13. Chiao, Y.-H.; Sengupta, A.; Chen, S.-T.; Huang, S.-H.; Hu, C.-C.; Hung, W.-S.; Chang, Y.; Qian, X.; Wickramasinghe, S.R.; Lee, K.-R.; et al. Zwitterion augmented polyamide membrane for improved forward osmosis performance with significant antifouling characteristics. Sep. Purif. Technol. 2019, 212, 316–325. [CrossRef] 14. Chiao, Y.-H.; Chen, S.-T.; Patra, T.; Hsu, C.-H.; Sengupta, A.; Hung, W.-S.; Huang, S.-H.; Qian, X.; Wickramasinghe, R.; Chang, Y. Zwitterionic forward osmosis membrane modified by fast second interfacial polymerization with enhanced antifouling and antimicrobial properties for produced water pretreatment. Desalination 2019, 469, 114090. [CrossRef] 15. Chen, G.Q.; Artemi, A.; Lee, J.; Gras, S.L.; Kentish, S.E. A pilot scale study on the concentration of milk and whey by forward osmosis. Sep. Purif. Technol. 2019, 215, 652–659. [CrossRef] 16. Wang, Y.-N.; Wang, R.; Li, W.; Tang, C.Y. Whey recovery using forward osmosis—Evaluating the factors limiting the flux performance. J. Membr. Sci. 2017, 533, 179–189. [CrossRef] 17. Li, B.; Japip, S.; Chung, T.-S. Molecularly tunable thin-film nanocomposite membranes with enhanced molecular sieving for organic solvent forward osmosis. Nat. Commun. 2020, 11, 1198. [CrossRef] 18. Cheng, Z.L.; Li, X.; Chung, T.-S. The forward osmosis-pressure retarded osmosis (FO-PRO) hybrid system: A new process to mitigate membrane fouling for sustainable osmotic power generation. J. Membr. Sci. 2018, 559, 63–74. [CrossRef] 19. Rastogi, N.K. Opportunities and challenges in application of forward osmosis in food processing. Crit. Rev. Food Sci. Nutr. 2016, 56, 266–291. [CrossRef] 20. Chun, Y.; Mulcahy, D.; Zou, L.; Kim, I.S. A Short Review of Membrane Fouling in Forward Osmosis Processes. Membranes 2017, 7, 30. [CrossRef] [PubMed] 21. Cui, Y.; Chung, T.-S. Pharmaceutical concentration using organic solvent forward osmosis for solvent recovery. Nat. Commun. 2018, 9, 1426. [CrossRef] 22. Dong, X.; Ge, Q. Metal Ion-Bridged Forward Osmosis Membranes for Efficient Pharmaceutical Wastewater Reclamation. Acs Appl. Mater. Interfaces 2019, 11, 37163–37171. [CrossRef] 23. Xiong, S.; Xu, S.; Zhang, S.; Phommachanh, A.; Wang, Y. Highly permeable and antifouling TFC FO membrane prepared with CD-EDA monomer for protein enrichment. J. Membr. Sci. 2019, 572, 281–290. [CrossRef] 24. Wu, W.; Shi, Y.; Liu, G.; Fan, X.; Yu, Y. Recent development of graphene oxide based forward osmosis membrane for water treatment: A critical review. Desalination 2020, 491, 114452. [CrossRef] 25. Siddiqui, F.A.; She, Q.; Fane, A.G.; Field, R.W. Exploring the differences between forward osmosis and reverse osmosis fouling. J. Membr. Sci. 2018, 565, 241–253. [CrossRef] 26. Yadav, S.; Saleem, H.; Ibrar, I.; Naji, O.; Hawari, A.A.; Alanezi, A.A.; Zaidi, S.J.; Altaee, A.; Zhou, J. Recent developments in forward osmosis membranes using carbon-based nanomaterials. Desalination 2020, 482, 114375. [CrossRef] 27. McGinnis, R.L.; Elimelech, M. Energy requirements of ammonia–carbon dioxide forward osmosis desalination. Desalination 2007, 207, 370–382. [CrossRef] 28. Achilli, A.; Cath, T.Y.; Marchand, E.A.; Childress, A.E. The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes. Desalination 2009, 239, 10–21. [CrossRef] 29. Mi, B.; Elimelech, M. Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents. J. Membr. Sci. 2010, 348, 337–345. [CrossRef] 30. Arjmandi, M.; Peyravi, M.; Altaee, A.; Arjmandi, A.; Pourafshari Chenar, M.; Jahanshahi, M.; Binaeian, E. A state-of-the-art protocol to minimize the internal concentration polarization in forward osmosis membranes. Desalination 2020, 480, 114355. [CrossRef] 31. Kahrizi, M.; Lin, J.; Ji, G.; Kong, L.; Song, C.; Dumée, L.F.; Sahebi, S.; Zhao, S. Relating forward water and reverse salt fluxes to membrane porosity and tortuosity in forward osmosis: CFD modelling. Sep. Purif. Technol. 2020, 241, 116727. [CrossRef] 32. Cath, T.Y.; Elimelech, M.; McCutcheon, J.R.; McGinnis, R.L.; Achilli, A.; Anastasio, D.; Brady, A.R.; Childress, A.E.; Farr, I.V.; Hancock, N.T.; et al. Standard Methodology for Evaluating Membrane Performance in Osmotically Driven Membrane Processes. Desalination 2013, 312, 31–38. [CrossRef] 33. Bui, N.-N.; Lind, M.L.; Hoek, E.M.V.; McCutcheon, J.R. Electrospun nanofiber supported thin film composite membranes for engineered osmosis. J. Membr. Sci. 2011, 385–386, 10–19. [CrossRef] 34. Bui, N.-N.; McCutcheon, J.R. Nanoparticle-embedded nanofibers in highly permselective thin-film nanocomposite membranes for forward osmosis. J. Membr. Sci. 2016, 518, 338–346. [CrossRef] 35. Liang, H.-Q.; Hung, W.-S.; Yu, H.-H.; Hu, C.-C.; Lee, K.-R.; Lai, J.-Y.; Xu, Z.-K. Forward osmosis membranes with unprecedented water flux. J. Membr. Sci. 2017, 529, 47–54. [CrossRef] 36. McCutcheon, J.R.; McGinnis, R.L.; Elimelech, M. A novel ammonia—carbon dioxide forward (direct) osmosis desalination process. Desalination 2005, 174, 1–11. [CrossRef] 37. Chang, H.; Li, T.; Liu, B.; Vidic, R.D.; Elimelech, M.; Crittenden, J.C. Potential and implemented membrane-based technologies for the treatment and reuse of flowback and produced water from shale gas and oil plays: A review. Desalination 2019, 455, 34–57. [CrossRef] Polymers 2021, 13, 583 17 of 18

38. Lee, W.J.; Ng, Z.C.; Hubadillah, S.K.; Goh, P.S.; Lau, W.J.; Othman, M.H.D.; Ismail, A.F.; Hilal, N. Fouling mitigation in forward osmosis and membrane distillation for desalination. Desalination 2020, 480, 114338. [CrossRef] 39. Yadav, S.; Ibrar, I.; Bakly, S.; Khanafer, D.; Altaee, A.; Padmanaban, V.C.; Samal, A.K.; Hawari, A.H. Organic Fouling in Forward Osmosis: A Comprehensive Review. Water 2020, 12, 1505. [CrossRef] 40. Chiao, Y.-H.; Chen, S.-T.; Ang, M.B.M.Y.; Patra, T.; Castilla-Casadiego, D.A.; Fan, R.; Almodovar, J.; Hung, W.-S.; Wickramasinghe, S.R. High-Performance Polyacrylic Acid-Grafted PVDF Nanofiltration Membrane with Good Antifoul- ing Property for the Textile Industry. Polymers 2020, 12, 2443. [CrossRef] 41. Zhang, G.; Zhan, Y.; He, S.; Zhang, L.; Zeng, G.; Chiao, Y.H. Construction of superhydrophilic/underwater superoleophobic polydopamine-modified h-BN/poly (arylene ether nitrile) composite membrane for stable oil-water emulsions separation. Polym. Adv. Technol. 2020, 31, 1007–1018. [CrossRef] 42. Zeng, G.; Wei, K.; Yang, D.; Yan, J.; Zhou, K.; Patra, T.; Sengupta, A.; Chiao, Y.-H. Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes. Colloids Surf. A Physicochem. Eng. Asp. 2019, 124142. [CrossRef] 43. She, Q.; Wang, R.; Fane, A.G.; Tang, C.Y. Membrane fouling in osmotically driven membrane processes: A review. J. Membr. Sci. 2016, 499, 201–233. [CrossRef] 44. Qi, L.; Hu, Y.; Liu, Z.; An, X.; Bar-Zeev, E. Improved Anti-Biofouling Performance of Thin -Film Composite Forward-Osmosis Membranes Containing Passive and Active Moieties. Environ. Sci. Technol. 2018, 52, 9684–9693. [CrossRef] 45. Sin, M.-C.; Chen, S.-H.; Chang, Y. Hemocompatibility of zwitterionic interfaces and membranes. Polym. J. 2014, 46, 436–443. [CrossRef] 46. Luk, Y.-Y.; Kato, M.; Mrksich, M. Self-assembled monolayers of alkanethiolates presenting mannitol groups are inert to protein adsorption and cell attachment. Langmuir 2000, 16, 9604–9608. [CrossRef] 47. Chiao, Y.-H.; Patra, T.; Ang, M.B.M.Y.; Chen, S.-T.; Almodovar, J.; Qian, X.; Wickramasinghe, R.; Hung, W.-S.; Huang, S.-H.; Chang, Y. Zwitterion Co-Polymer PEI-SBMA Nanofiltration Membrane Modified by Fast Second Interfacial Polymerization. Polymers 2020, 12, 269. [CrossRef] 48. Schlenoff, J.B. Zwitteration: Coating Surfaces with Zwitterionic Functionality to Reduce Nonspecific Adsorption. Langmuir 2014, 30, 9625–9636. [CrossRef] 49. Lin, W.; Klein, J. Control of surface forces through hydrated boundary layers. Curr. Opin. Colloid Interface Sci. 2019, 44, 94–106. [CrossRef] 50. Erfani, A.; Seaberg, J.; Aichele, C.P.; Ramsey, J.D. Interactions between Biomolecules and Zwitterionic Moieties: A Review. Biomacromolecules 2020, 21, 2557–2573. [CrossRef][PubMed] 51. Chiao, Y.-H.; Chen, S.-T.; Sivakumar, M.; Ang, M.B.M.Y.; Patra, T.; Almodovar, J.; Wickramasinghe, S.R.; Hung, W.-S.; Lai, J.-Y. Zwitterionic Polymer Brush Grafted on Polyvinylidene Difluoride Membrane Promoting Enhanced Ultrafiltration Performance with Augmented Antifouling Property. Polymers 2020, 12, 1303. [CrossRef] 52. Chiao, Y.-H.; Ang, M.B.M.Y.; Huang, Y.-X.; DePaz, S.S.; Chang, Y.; Almodovar, J.; Wickramasinghe, S.R. A “Graft to” Electrospun Zwitterionic Bilayer Membrane for the Separation of Hydraulic Fracturing-Produced Water via Membrane Distillation. Membranes 2020, 10, 402. [CrossRef] 53. Han, L.; Tan, Y.Z.; Xu, C.; Xiao, T.; Trinh, T.A.; Chew, J.W. Zwitterionic grafting of sulfobetaine methacrylate (SBMA) on hydrophobic PVDF membranes for enhanced anti-fouling and anti-wetting in the membrane distillation of oil emulsions. J. Membr. Sci. 2019, 588, 117196. [CrossRef] 54. Zhou, Q.; Lei, X.-P.; Li, J.-H.; Yan, B.-F.; Zhang, Q.-Q. Antifouling, adsorption and reversible flux properties of zwitterionic grafted PVDF membrane prepared via physisorbed free radical polymerization. Desalination 2014, 337, 6–15. [CrossRef] 55. Guo, W.; Ngo, H.H.; Li, J. A mini-review on membrane fouling. Bioresour. Technol. 2012, 122, 27–34. [CrossRef] 56. Shon, H.K.; Vigneswaran, S.; Kim, I.S.; Cho, J.; Ngo, H.H. Fouling of ultrafiltration membrane by effluent organic matter: A detailed characterization using different organic fractions in wastewater. J. Membr. Sci. 2006, 278, 232–238. [CrossRef] 57. Yuan, W.; Zydney, A.L. Humic Acid Fouling during Ultrafiltration. Environ. Sci. Technol. 2000, 34, 5043–5050. [CrossRef] 58. Parida, V.; Ng, H.Y. Forward osmosis organic fouling: Effects of organic loading, calcium and membrane orientation. Desalination 2013, 312, 88–98. [CrossRef] 59. Mi, B.; Elimelech, M. Silica scaling and scaling reversibility in forward osmosis. Desalination 2013, 312, 75–81. [CrossRef] 60. Mi, B.; Elimelech, M. Gypsum Scaling and Cleaning in Forward Osmosis: Measurements and Mechanisms. Environ. Sci. Technol. 2010, 44, 2022–2028. [CrossRef] 61. Chun, Y.; Zaviska, F.; Kim, S.-J.; Mulcahy, D.; Yang, E.; Kim, I.S.; Zou, L. Fouling characteristics and their implications on cleaning of a FO-RO pilot process for treating brackish surface water. Desalination 2016, 394, 91–100. [CrossRef] 62. Li, Z.-Y.; Yangali-Quintanilla, V.; Valladares-Linares, R.; Li, Q.; Zhan, T.; Amy, G. Flux patterns and membrane fouling propensity during desalination of seawater by forward osmosis. Water Res. 2012, 46, 195–204. [CrossRef] 63. Flemming, H.C.; Schaule, G.; Griebe, T.; Schmitt, J.; Tamachkiarowa, A. Biofouling—the Achilles heel of membrane processes. Desalination 1997, 113, 215–225. [CrossRef] 64. Veza, J.M.; Ortiz, M.; Sadhwani, J.J.; Gonzalez, J.E.; Santana, F.J. Measurement of biofouling in seawater: Some practical tests. Desalination 2008, 220, 326–334. [CrossRef] Polymers 2021, 13, 583 18 of 18

65. Saeed, M.O.; Jamaluddin, A.T.; Tisan, I.A.; Lawrence, D.A.; Al-Amri, M.M.; Chida, K. Biofouling in a seawater reverse osmosis plant on the Red Sea coast, Saudi Arabia. Desalination 2000, 128, 177–190. [CrossRef] 66. Goulter, R.M.; Gentle, I.R.; Dykes, G.A. Issues in determining factors influencing bacterial attachment: A review using the attachment of Escherichia coli to abiotic surfaces as an example. Lett. Appl. Microbiol. 2009, 49, 1–7. [CrossRef][PubMed] 67. Ibrar, I.; Naji, O.; Sharif, A.; Malekizadeh, A.; Alhawari, A.; Alanezi, A.A.; Altaee, A. A Review of Fouling Mechanisms, Control Strategies and Real-Time Fouling Monitoring Techniques in Forward Osmosis. Water 2019, 11, 695. [CrossRef] 68. Singh, G.; Song, L. Experimental correlations of pH and ionic strength effects on the colloidal fouling potential of silica nanoparticles in crossflow ultrafiltration. J. Membr. Sci. 2007, 303, 112–118. [CrossRef] 69. Cohen, R.; Probstein, R. Colloidal fouling of reverse osmosis membranes. J. Colloid Interface Sci. 1986, 114, 194–207. [CrossRef] 70. Boo, C.; Lee, S.; Elimelech, M.; Meng, Z.; Hong, S. Colloidal fouling in forward osmosis: Role of reverse salt diffusion. J. Membr. Sci. 2012, 390–391, 277–284. [CrossRef] 71. Dizon, G.V.; Venault, A. Direct in-situ modification of PVDF membranes with a zwitterionic copolymer to form bi-continuous and fouling resistant membranes. J. Membr. Sci. 2018, 550, 45–58. [CrossRef] 72. Lee, W.J.; Goh, P.S.; Lau, W.J.; Ong, C.S.; Ismail, A.F. Antifouling zwitterion embedded forward osmosis thin film composite membrane for highly concentrated oily wastewater treatment. Sep. Purif. Technol. 2019, 214, 40–50. [CrossRef] 73. Romero-Vargas Castrillón, S.; Lu, X.; Shaffer, D.L.; Elimelech, M. Amine enrichment and poly(ethylene glycol) (PEG) surface modification of thin-film composite forward osmosis membranes for organic fouling control. J. Membr. Sci. 2014, 450, 331–339. [CrossRef] 74. Zhang, X.; Tian, J.; Gao, S.; Zhang, Z.; Cui, F.; Tang, C.Y. In situ surface modification of thin film composite forward osmosis membranes with sulfonated poly (arylene ether sulfone) for anti-fouling in emulsified oil/water separation. J. Membr. Sci. 2017, 527, 26–34. [CrossRef] 75. Venault, A.; Chang, Y. Designs of Zwitterionic Interfaces and Membranes. Langmuir 2019, 35, 1714–1726. [CrossRef] 76. Chen, Y.; Ge, Q. A Bifunctional Zwitterion That Serves as Both a Membrane Modifier and a Draw Solute for Forward Osmosis Wastewater Treatment. Acs Appl. Mater. Interfaces 2019, 11, 36118–36129. [CrossRef] 77. Zou, S.; Smith, E.D.; Lin, S.; Martin, S.M.; He, Z. Mitigation of bidirectional solute flux in forward osmosis via membrane surface coating of zwitterion functionalized carbon nanotubes. Environ. Int. 2019, 131, 104970. [CrossRef] 78. Nguyen, A.; Azari, S.; Zou, L. Coating zwitterionic amino acid l-DOPA to increase fouling resistance of forward osmosis membrane. Desalination 2013, 312, 82–87. [CrossRef] 79. Carter, B.M.; Sengupta, A.; Qian, X.; Ulbricht, M.; Wickramasinghe, S.R. Controlling external versus internal pore modification of ultrafiltration membranes using surface-initiated AGET-ATRP. J. Membr. Sci. 2018, 554, 109–116. [CrossRef] 80. Tripathi, T.; Kamaz, M.; Wickramasinghe, S.R.; Sengupta, A. Designing Electric Field Responsive Ultrafiltration Membranes by Controlled Grafting of Poly (Ionic Liquid) Brush. Int. J. Environ. Res. Public Health 2020, 17, 271. [CrossRef][PubMed] 81. Sengupta, A.; Wickramasinghe, R. Activator Generated Electron Transfer Combined Atom Transfer Radical Polymerization (AGET-ATRP) for Controlled Grafting Location of Glycidyl Methacrylate on Regenerated Cellulose Ultrafiltration Membranes. J. Membr. Sci. Res. 2020, 6, 90–98. 82. Liu, C.; Lee, J.; Ma, J.; Elimelech, M. Antifouling Thin-Film Composite Membranes by Controlled Architecture of Zwitterionic Polymer Brush Layer. Environ. Sci. Technol. 2017, 51, 2161–2169. [CrossRef][PubMed] 83. Liu, C.; Lee, J.; Small, C.; Ma, J.; Elimelech, M. Comparison of organic fouling resistance of thin-film composite membranes modified by hydrophilic silica nanoparticles and zwitterionic polymer brushes. J. Membr. Sci. 2017, 544, 135–142. [CrossRef] 84. Zhang, X.; Gao, S.; Tian, J.; Shan, S.; Takagi, R.; Cui, F.; Bai, L.; Matsuyama, H. Investigation of Cleaning Strategies for an Antifouling Thin-Film Composite Forward Osmosis Membrane for Treatment of Polymer-Flooding Produced Water. Ind. Eng. Chem. Res. 2019, 58, 994–1003. [CrossRef] 85. Ju, C.; Kang, H. Zwitterionic polymers showing upper critical solution temperature behavior as draw solutes for forward osmosis. Rsc Adv. 2017, 7, 56426–56432. [CrossRef] 86. Lutchmiah, K.; Lauber, L.; Roest, K.; Harmsen, D.J.H.; Post, J.W.; Rietveld, L.C.; van Lier, J.B.; Cornelissen, E.R. Zwitterions as alternative draw solutions in forward osmosis for application in wastewater reclamation. J. Membr. Sci. 2014, 460, 82–90. [CrossRef]